SYRACUSE, N.Y. -The automatic shutdown of Nine Mile Point Unit 1 nuclear reactor during Superstorm Sandy would not have occurred but for the failure of plant personnel to make sure a transformer was installed properly more than a year earlier, federal regulators say.

Unit 1, one of three nuclear plants in Oswego County, shut down the night of Oct. 29 after wind gusts from the big storm toppled a tall pole called a lightning arrestor in a nearby National Grid switchyard. The collapsed pole caused an electrical fault on one of two lines that carry power from the nuclear plant out to the grid.

According to a report issued Monday by the U.S. Nuclear Regulatory Commission, the switchyard accident should not have caused the nuclear plant to shut down, or scram. The scram happened because a sensor in a transformer owned by the nuclear plant was configured improperly when it was installed in April 2011, NRC officials said.

The improperly installed sensor, which was supposed to monitor the circuit between the plant and the transformer, instead monitored activity in the switchyard. When the sensor detected a fault, it shut the plant down.

The finding was included in a NRC inspection report that covered plant operations during the last quarter of 2012. Regulators criticized plant owner Constellation Energy Nuclear Group for failing to properly oversee the work of a third-party vendor that installed the transformer. But NRC officials said the scram itself had “very low safety significance’’ because it did not impair safety equipment needed to return the plant to a stable condition.

Constellation restarted Unit 1 four days after the storm-related scram, but was forced to shut it down again the next day after a separate problem developed in a component that controls the water level in the reactor. After another week out of commission, the plant resumed operation.

Jill Lyon, speaking for Constellation, said company personnel identified the problem with the transformer installation and have revised procedures to prevent similar issues from arising. "We've taken the appropriate action to prevent it from occurring again,'' she said.

Monday’s inspection report cited Constellation for a second problem during the quarter, in which a control room emergency ventilation system that had been shut down for troubleshooting was returned to service before the problem was fully corrected. That finding also was considered of “very low safety significance,’’ the NRC said.